Scanning electron microscopes (SEMs) are common tools for the observation of small features. In this technique, a finely focused electron beam (“probe”) with an energy typically between about 1 keV and 20 keV is scanned across the surface of a sample. At each probe position on the surface, signals are conventionally measured by a number of different types of detector. Commonly, scattered electrons and x-rays are detected and for each signal, magnified images of the specimen are constructed where the signal strength modulates the intensity at positions corresponding to the probe positions scanned on the surface. At each probe position, the energy spectrum of the scattered electrons and x-rays is determined by the composition and topography of the sample surface and the underlying bulk material. At present compositional analysis in SEMs is most commonly achieved with x-ray detection.
U.S. Pat. No. 4,559,450 describes an analytical technique that makes use of the fact that the total intensity of backscattered electrons (BE) is affected by the atomic number of the specimen. Backscattered electrons are high-energy electrons which are back-scattered from the specimen as a result of its interaction with the beam. Whilst it is secondary electrons rather than backscattered electrons that are used in many imaging applications, backscattered electrons are advantageous since their energy can be analysed so as to obtain some information upon the composition of the part of the material with which the electron beam interacts.
If the BE signal intensity is compared to the intensity from a reference material for the same incident beam current, it is possible to decide if the specimen has an effective atomic number higher or lower than the reference material. A calibration curve can be constructed from a series of reference materials so that the effective atomic number of an unknown specimen can be determined from the BE signal intensity. Furthermore, the effective atomic number for backscattered electrons can be predicted from the chemical composition so that the observed signal can be used to corroborate a “guessed” composition for the material. The guessed composition can be obtained by using x-ray analysis of the specimen to provide information on relative elemental concentrations.
One of the problems of analysis with electrons is that energetic electrons scatter within the specimen and penetrate below the surface. Therefore, x-rays are generated throughout the region reached by the electrons. For example a 15 keV incident beam generates Si K x-rays from a depth of about 3000 nm in pure silicon and a lateral range of a similar dimension. Therefore in these conditions, spatial resolution of analytical information from x-rays is of the order of 3000 nm. For example FIG. 1 shows a 1000 nm diameter particle 10 of silicon on top of a carbon substrate 11 and shows that many electrons reach the substrate where they will generate x-rays that are not representative of the particle. Within FIG. 1, the incident focussed electron beam is shown at 1. Electrons which are scattered back out into the vacuum as backscattered electrons (BE) are shown at 2. X-rays are generated from the electron trajectories within the particle 10, these being illustrated at 3. The electron trajectories within the substrate 11 generate x-rays from the substrate. The electrons are scattered out of the particle 10 so that the x-ray signal is a combination of the signal from the particle 10 and the signal from the substrate 11. Note that in FIG. 1, the vertical scale is in micrometre units.
To contribute to the BE signal, electrons have to scatter backwards out of the specimen and reach a BE detector. Most of these electrons come from depths less than 1000 nm and from a similar lateral range. Therefore, in these conditions, the analytical information provided by the BE signal has a spatial resolution of the order of 1000 nm. As in the case of x-ray analysis, if an object is smaller than the analytical spatial resolution, then the signal generated by the object will not be truly representative of the object material. Therefore, the method based on backscattered electrons described by U.S. Pat. No. 4,559,450 would typically not be suitable to analyse objects much smaller than 1000 nm in dimension using 15 keV incident electrons.
U.S. Pat. No. 6,753,525 recognises that, for small objects, the BE count will depend on the physical dimension of the object being studied and uses a look-up table showing the normalised backscattered electron count as a function of both effective atomic number of the material and the physical dimension of the object being studied. U.S. Pat. No. 6,753,525 also recognises that the high energy of the incident electron beam causes x-rays to emanate from the areas surrounding the features of interest and the size of the region emitting x-rays is considerably larger than for the region producing backscattered electrons. Consequently, the resulting x-ray spectrum cannot be corrected for feature size using a table of correction factors and there is no provision for identifying chemical elements in U.S. Pat. No. 6,753,525. Furthermore, U.S. Pat. No. 6,753,525 makes no reference to the effect of the substrate beneath a small object on the observed BE signal. If the object is partially transparent to incident electrons then the total BE signal will depend on the both the material of the object, the dimensions of the object and the material of the substrate beneath.
For a practical implementation of U.S. Pat. No. 6,753,525 it is likely that the substrate is silicon because the application is to analyse small objects on a semiconductor wafer, which is commonly silicon. However, if the substrate were changed, the lookup table relating BE signal to object dimensions would have to be modified to compensate for the different electron scattering properties of a substrate other than silicon, this requiring significant additional effort.
In both U.S. Pat. No. 4,559,450 and U.S. Pat. No. 6,753,525 the total BE signal is used for characterisation. The BE detectors used in SEMs are typically solid state detectors, scintillator/photomultiplier detectors and micro-channel plate detectors and are used in “analog” mode where there is a continuous readout of the aggregate current liberated in the detector. In a solid state detector, the charge liberated by a single electron striking the detector is roughly proportional to the energy of the incident electron and therefore the total measured current will not only depend on the rate of backscattered electrons hitting the detector but also the spectral distribution of the backscattered electrons. Similarly, all types of BE detector based on electrical current measurement produce a result that depends to some extent on the energy spectrum of incident electrons as well as the count rate of incident electrons. In order to calculate the theoretical expected BE signal from a material, it is therefore necessary to characterise the exact energy response of the BE detector. When there are no electrons hitting the detector, the detector electronics registers an offset current and this may vary with time. Therefore, for any calibration method it is important to compensate for the offset current prior to each measurement. Furthermore, the measured current also depends on the gain of the electronic amplification system and this may vary. The energy dependence of detectors, variation in offset current and variation in gain make characterisation of materials by backscattered electrons difficult in practice.
There are therefore a number of problems associated with the use of backscattered electrons and x-ray techniques in obtaining compositional information, particularly where analysis is needed of objects having small dimensions, such as less than 1000 nanometres.